The amount of fine paper produced under alkaline conditions has been increasing rapidly, encouraged by cost savings, the ability to use precipitated calcium carbonate (PCC), an increased demand for improved paper permanence and brightness, and an increased tendency to close the wet-end of the paper machine.
Current applications for fine paper require particular attention to sizing before conversion or end-use, such as high-speed photocopies, envelopes, forms bond including computer printer paper, and adding machine paper. The most common sizing agents for fine paper made under alkaline conditions are alkenyl succinic anhydride (ASA) and alkyl ketene dimer (AKD). Both types of sizing agents have a reactive functional group that covalently bonds to cellulose fiber and hydrophobic tails that are oriented away from the fiber. The nature and orientation of these hydrophobic tails cause the fiber to repel-water.
Commercial AKD's, containing one .beta.-lactone ring, are prepared by the dimerization of the alkyl ketenes made from two saturated, straight-chain fatty acid chlorides; the most widely used being prepared from palmitic and/or stearic acid. Other ketene dimers, such as the alkenyl based ketene dimer (Aquapel.RTM. 421 of Hercules Incorporated), have also been used commercially. Ketene multimers, containing more than one such .beta.-lactone ring, have been described in Japanese Kokai 168992/89, the disclosure of which is incorporated herein by reference. ASA-based sizing agents may be prepared by the reaction of maleic anhydride with an olefin (C.sub.14 -C.sub.18).
Although ASA and AKD sizing agents are commercially successful, they have disadvantages. Both types of sizing agents, particularly the AKD type, have been associated with handling problems in the typical high-speed conversion operations required for the current uses of fine paper made under alkaline conditions (referred to as alkaline fine paper). The problems include reduced operating speed in forms presses and other converting machines, double feeds or jams in high-speed copiers, and paper-welding and registration errors on printing and envelope-folding equipment that operates at high speeds.
These problems are not normally associated with fine paper produced under acid conditions (acid fine paper). The types of filler and filler addition levels used to make alkaline fine paper differ significantly from those used to make acid fine paper, and can cause differences in paper properties such as stiffness and coefficient of friction which affect paper handling. Alum addition levels in alkaline fine paper, which contribute to sheet conductivity and dissipation of static charge, also differ significantly from those used in acid fine paper. This is important because the electrical properties of paper affect its handling performance. Sodium chloride is often added to the surface of alkaline fine paper to improve its performance in end use.
The typical problems encountered with the conversion and end-use handling of alkaline fine paper involve:
1. Paper properties related to composition of the furnish;
2. Paper properties developed during paper formation; and
3. Problems related to sizing.
The paper properties affected by paper making under alkaline conditions that can affect converting and end-use performance include:
Curl
Variation In Coefficient Of Friction
Moisture Content
Moisture Profile
Stiffness
Dimensional Stability
MD/CD Strength Ratios
One such problem has been identified and measured as described in "Improving The Performance Of Alkaline Fine Paper On The IBM 3800 Laser Printer," TAPPI Paper Makers Conference Proceedings (1991), the disclosure of which is incorporated herein by reference. The problem occurs when using an IBM 3800 high speed continuous forms laser printer that does not have special modifications intended to facilitate handling of alkaline fine paper. That commercially-significant laser printer therefore can serve as an effective testing device for defining the convertibility of various types of sized paper on state-of-the-art converting equipment and its subsequent end-use performance. In particular, the phenomenon of "billowing" gives a measurable indication of the extent of slippage on the IBM 3800 printer between the undriven roll beyond the fuser and the driven roll above the stacker.
Such billowing involves a divergence of the paper path from the straight line between the rolls, which is two inches above the base plate, causing registration errors and dropped folds in the stacker. The rate of billowing during steady-state running time is measured as the billowing height in inches above the straight paper path after 600 seconds of running time and multiplied by 10,000.
Typical alkaline AKD sized fine paper using a size furnish of 2.2 lbs. per ton of paper shows an unacceptable rate-of-billowing, typically of the order of 20 to 80. Paper handling rates on other high-speed converting machinery, such as a Hamilton-Stevens continuous forms press or a Winkler & Dunnebier CH envelope folder, also provide numerical measures of convertibility.
There is a need for alkaline fine paper that provides improved handling performance in typical converting and reprographic operations. At the same time, the levels of sizing development need to be comparable to that obtained with the current furnish levels of AKD or ASA for alkaline fine paper.